Optical mice are commonly used as navigation input devices for computers. Conventional optical mice are generally categorized by the type of light source that is used. Some optical mice use a light emitting diode (LED), while other optical mice use a laser. The light source typically illuminates a navigation surface such as a desktop. Light that reflects off of the navigation surface is sensed, or imaged, by an image sensor. The image sensor generates electrical signals representative of the light intensities incident on the image sensor. Using sequential images, a tracking engine can determine the relative movement between the optical mouse and the navigation surface.
Simple relay lenses with typically near 1:1 magnification are commonly used in both LED and laser optical mice to direct reflected light from the navigation surface to the image sensor. Such lenses are typically made of molded plastic in known designs. These conventional lenses are commonly used to image a portion of the navigation surface that is centered on the optical axis of the lens. By way of comparison, the light source is off the lens axis so that direct light does not enter the lens. Rather, scattered light reflected from the navigation surface enters the lens and allows the image sensor to form the images for navigation applications.
In some conventional systems, the lens is tilted relative to the navigation surface so that the lens can receive light from the direct specular reflection off of the navigation surface. In these types of conventional systems, distortion and depth of field issues may affect the accuracy of the lens system, so these types of lenses are often “stopped down” to get adequate depth of field.
In other conventional embodiments, the image sensor is at an oblique angle relative to the navigation surface. In this configuration, the specular reflected beam is deliberately excluded from the lens, so that only light that scatters at small angles about the specular beam is accepted by the lens system. However, this type of imaging lens is typically quite large, so that the lens is relatively fast and can create images of good quality to minimize leakage of the specular beam into the imaging lens. This precludes tilting the lens viewing or optical axis relative to the normal of the navigation surface, yet a widely eccentric field is viewed that is off the axis of the aperture stop of the system by a central field angle of 22 degrees of more.
This type of configuration would typically call for a wide field angle lens, with a good image field of view of over 50 degrees diameter, yet it forms an undistorted image of the eccentric field with sharp image quality. In an optical mouse, most of the useful field of view of such a wide field angle lens is not utilized. Consequently, this type of conventional configuration would be costly to implement.
It should also be noted that this type of configuration might be implemented using a Scheimflug arrangement. In a Scheimflug arrangement, the object plane and the image plane are at a first angle relative to each other, and the lens is at some other angle. In some arrangements, the lens and the aperture stop have a common optical axis, which is horizontal so the lens is substantially vertical. In some embodiments, the object plane and the image plane of the Scheimflug geometry intersect on the plane of the lens.
Although this arrangement may provide an image that is substantially in focus on the tilted image plane, certain illuminated portions may have a higher magnification. In this way, the magnification changes over the image surface, so that the image exhibits pronounced “keystone” distortion of rectangular objects. Hence, the conventional Scheimflug arrangement is less than ideal because of the keystone distortion.